Memory device

ABSTRACT

The present invention provides a memory device in which lower electrodes, a buffer layer, a seed layer, a magnetic tunnel junction, a capping layer, synthetic exchange diamagnetic layers, and an upper electrode are formed on a substrate in a laminated manner. According to the present invention, the lower electrodes and the seed layer are formed of a polycrystalline conductive material, and the perpendicular magnetic anisotropy of the magnetic tunnel junction is maintained upon heat treatment at a high temperature of 400° C. or more.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/KR2015/002607 filed Mar. 18, 2015 which claims the priority benefitof Korean Patent Application No. 10-2014-0046563, filed on Apr. 18,2014, and Korean Patent Application No. 10-2014-0102419, filed on Aug.8, 2014, in the Korean Intellectual Property Office, the disclosure ofeach of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a memory device, and more particularly,to a magnetic memory device including a magnetic tunnel junction (MTJ).

BACKGROUND ART

Next-generation non-volatile memory devices with lower power consumptionand higher degree of integration than flash memory devices are beingstudied. Such next-generation non-volatile memory devices includephase-change random access memory (PRAM) that uses state changes of aphase change material such as chalcogenide alloys, magnetic randomaccess memory (MRAM) that uses resistance changes in a magnetic tunneljunction (MTJ) depending on the magnetization state of a ferromagneticmaterial, ferroelectric random access memory (FRAM) that usespolarization of a ferroelectric material, resistance-change randomaccess memory (ReRAM) that uses resistance changes in a variableresistance material, and the like.

Examples of MRAM include a spin-transfer torque magnetic random accessmemory (STT-MRAM) device that inverts magnetization using aspin-transfer torque (STT) phenomenon generated by electron injectionand discriminates a resistance difference before and after magnetizationinversion. The STT-MRAM device includes a magnetic tunnel junction,which consists of a pinned layer and a free layer, each formed of aferromagnetic material, and a tunnel barrier disposed therebetween. Inthe magnetic tunnel junction, when the magnetization directions of thefree layer and the pinned layer are the same (that is, parallel),current flow is easy and consequently the magnetic tunnel junction is ina low resistance state. On the other hand, when the magnetizationdirections are different (that is, antiparallel), current is reduced andconsequently the magnetic tunnel junction is in a high resistance state.In addition, in the magnetic tunnel junction, the magnetizationdirections must change only in the direction perpendicular to asubstrate. Therefore, the free layer and the pinned layer must haveperpendicular magnetization values. When the perpendicular magnetizationvalues are symmetrical with respect to 0 according to the intensity anddirection of a magnetic field, and a squareness (S) shape becomes clear(S=1), perpendicular magnetic anisotropy (PMA) is considered to beexcellent. The STT-MRAM device is theoretically capable of cycling morethan 10¹⁵ times and can be switched at a high speed of about a fewnanoseconds (ns). In particular, a perpendicular magnetization typeSTT-MRAM device is advantageous in that there is no theoretical scalinglimit, and as scaling progresses, the current density of driving currentmay be lowered. Therefore, the perpendicular magnetization type STT-MRAMdevice has been actively studied as a next-generation memory device thatmay replace DRAM devices. An example of the STT-MRAM device is disclosedin Korean Patent No. 10-1040163.

In the STT-MRAM device, a seed layer is formed on the lower part of thefree layer, a capping layer is formed on the upper part of the pinnedlayer, and synthetic antiferromagnetic (SyAF) layers and an upperelectrode are formed on the upper part of the capping layer. Inaddition, in the STT-MRAM device, a silicon oxide film is formed on asilicon substrate, and then the seed layer and a magnetic tunneljunction are formed thereon. In addition, a selection element such as atransistor may be formed on the silicon substrate, and the silicon oxidefilm may be formed so as to cover the selection element. Therefore, theSTT-MRAM device has a laminated structure in which a silicon oxide film,a seed layer, a free layer, a tunnel barrier, a pinned layer, a cappinglayer, SyAF layers, and an upper electrode are formed on a siliconsubstrate on which a selection element is formed. In this case, the seedlayer and the capping layer are formed using tantalum (Ta), and the SyAFlayers have a structure in which a lower magnetic layer and an uppermagnetic layer, in which a magnetic metal and a non-magnetic metal arealternately stacked, are formed, and a non-magnetic layer is formedtherebetween.

However, since the seed layer formed on the upper part of the amorphoussilicon oxide film is amorphous and the magnetic tunnel junction is alsoamorphous, the crystallinity of the magnetic tunnel junction may bedeteriorated. That is, the pinned layer and the free layer are formed ofamorphous CoFeB. In this case, the crystallinity of the magnetic tunneljunction is not greatly improved even when heat treatment is performedto improve perpendicular anisotropy characteristics. When thecrystallinity of the magnetic tunnel junction is low, perpendicularmagnetic anisotropy may be lowered. Therefore, even when a magneticfield is applied to change the magnetization direction, themagnetization direction does not change rapidly, and the amount ofcurrent flowing in a parallel state is reduced. As a result, read/writetime is delayed, which makes it difficult to implement a high-speedmemory device, and operation errors may occur during read/writeoperation.

In addition, a metal line forming process and a passivation process areperformed after formation of the SyAF layers and the upper electrode. Atthis time, these processes are performed at a temperature of about 400°C. However, when Ta is used as the seed layer, the perpendicularmagnetic anisotropy of the magnetic tunnel junction is lowered at atemperature of about 400° C. Therefore, it is necessary to improve thethermal stability of the perpendicular magnetic anisotropy of themagnetic tunnel junction.

DISCLOSURE Technical Problem

Therefore, the present invention has been made in view of the aboveproblems, and it is one object of the present invention to provide amemory device capable of rapidly changing the magnetization direction ofa magnetic tunnel junction and thereby increasing read/write operationspeed.

It is another object of the present invention to provide a memory devicecapable of improving the crystallinity of a magnetic tunnel junction andthereby rapidly changing the magnetization direction of the magnetictunnel junction.

It is yet another object of the present invention to provide a memorydevice capable of improving the thermal stability of the perpendicularmagnetic anisotropy of a magnetic tunnel junction.

Technical Solution

In accordance with one aspect of the present invention, provided is amemory device in which lower electrodes, a buffer layer, a seed layer, amagnetic tunnel junction, a capping layer, SyAF layers, and an upperelectrode are formed on a substrate in a laminated manner, wherein themagnetic tunnel junction includes free layers consisting of first andsecond free layers and a separation layer, a tunneling barrier and apinned layer, wherein the separation layer is formed between the firstand second free layers and is formed of a polycrystalline conductivematerial.

The memory device may further include a lower barrier formed between theseed layer and the first free layer.

The lower barrier may be formed of a material identical to that of thetunneling barrier.

The separation layer may be formed of a material which crystallizes at atemperature of 400° C. or higher.

The separation layer may be formed of tungsten.

The separation layer may be formed to have a thickness of 0.4 nm to 0.8nm.

The first and second free layers may be formed to have the samethicknesses or different thicknesses.

At least one of the lower electrodes, the seed layer, and the cappinglayer may be formed of a material which crystallizes at a temperature of400° C. or higher.

At least one of the lower electrodes, the seed layer, and the cappinglayer may be formed of tungsten.

The magnetic tunnel junction may have a squareness of magnetizationchange of more than 0.8 and 1 or less after heat treatment at 400° C. orhigher.

In accordance with another aspect of the present invention, provided isa memory device in which lower electrodes, a buffer layer, a seed layer,a magnetic tunnel junction, a capping layer, SyAF layers, and an upperelectrode are formed on a substrate in a laminated manner, wherein themagnetic tunnel junction includes free layers consisting of first andsecond free layers and a separation layer, a tunneling barrier and apinned layer, wherein the separation layer is formed between the firstand second free layers; and at least one of the lower electrodes, theseed layer, the separation layer, and the capping layer is formed of apolycrystalline conductive material.

The polycrystalline conductive material may include a material whichcrystallizes at a temperature of 400° C. or higher.

The polycrystalline conductive material may include tungsten.

In accordance with yet another aspect of the present invention, providedis a method of fabricating a memory device, including a step of forminglower electrodes, a buffer layer, a seed layer, a magnetic tunneljunction, a capping layer, SyAF layers, and an upper electrode on asubstrate in a laminated manner; and a step of performing heat treatmentat 400° C. to 500° C., wherein, when the magnetic tunnel junction isformed, a first free layer, a separation layer, and a second free layerare formed and then a tunneling barrier and a pinned layer are formed,wherein the separation layer is formed of a polycrystalline conductivematerial; and the heat treatment is performed after the upper electrodeis formed.

Advantageous Effects

In the memory device of the present invention, lower electrodes areformed of a polycrystalline material, a seed layer formed of apolycrystalline material is formed thereon, and a buffer layer is formedbetween the lower electrodes and the seed layer to improve compatibilitybetween the lower electrodes and the seed layer. Since the lowerelectrodes and the seed layer are formed of a polycrystalline material,an amorphous magnetic tunnel junction formed on thereon is aligned withthe crystal structure of the seed layer. In addition, by heat treatmentafter layer formation, an improved crystal structure can be obtained ascompared with the conventional device. Therefore, the memory device canrapidly change the magnetization direction of the magnetic tunneljunction, thereby increasing read/write operation speed.

In addition, in the memory device of the present invention, the freelayer can be formed to have a structure including a first free layer, aseparation layer, and a second free layer so that the thickness of thefree layer increases, thereby improving the thermal stability of thememory device. In addition, increase in the thickness of the free layerdue to the presence of the separation layer can prevent horizontalmagnetic anisotropy.

In addition, according to the present invention, since the seed layerand the separation layer are formed using a metal including tungsten,the magnetic tunnel junction can maintain perpendicular magneticanisotropy even at a high temperature of 400° C. or more.

DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a memory device according to oneembodiment of the present invention.

FIG. 2 is a graph showing heat treatment temperature-dependentmagnetization of perpendicular magnetic anisotropy structures in which aseed layer is formed using Ta and W, respectively.

FIGS. 3 and 4 are graphs showing magnetic field-dependent magnetizationof perpendicular magnetic anisotropy structures in which a seed layer isformed using Ta and W, respectively.

FIGS. 5 to 7 are TEM images showing the crystal states of a magnetictunnel junction formed on a Ta seed layer according to a comparativeexample after the magnetic tunnel junction is heat-treated at varioustemperatures.

FIG. 8 is a graph showing heat treatment temperature-dependentmagnetization of a perpendicular magnetic anisotropy structure in whicha W seed layer is formed on a TiN lower electrode according to acomparative example.

FIG. 9 is a graph showing heat treatment temperature-dependentmagnetization of a perpendicular magnetic anisotropy structure in whicha Ta buffer layer and a W seed layer are formed on a W/TiN lowerelectrode according to an embodiment of the present invention.

FIG. 10 is an image of a magnetic tunnel junction formed on a TiN lowerelectrode and a Ta seed layer according to a comparative example afterheat treatment.

FIG. 11 is a TEM image of a magnetic tunnel junction formed on a TiNlower electrode, a Ta buffer layer, and a W seed layer according to anembodiment of the present invention after heat treatment.

FIG. 12 is a graph showing magnetic field-dependent magnetization of apseudo spin valve in which a W/TiN lower electrode, a Ta buffer layer, aW seed layer, and a W capping layer are formed according to anembodiment of the present invention.

FIG. 13 is a cross-sectional view of a memory device according toanother embodiment of the present invention.

BEST MODE

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.However, the present invention may be embodied in many different formsand should not be construed as limited to the embodiments set forthherein. Rather, these embodiments are provided so that this disclosurewill be thorough and complete, and will fully convey the scope of thedisclosure to those skilled in the art.

FIG. 1 is a cross-sectional view of a memory device according to oneembodiment of the present invention, that is, a cross-sectional view ofan STT-MRAM device.

Referring to FIG. 1, a memory device according to one embodiment of thepresent invention includes lower electrodes 110, a buffer layer 120, aseed layer 130, a free layer 140, a tunneling barrier 150, a pinnedlayer 160, a capping layer 170, SyAF layers 180, and an upper electrode190, which are formed on a substrate 100. In this case, the free layer140, the tunneling barrier 150, and the pinned layer 160 form a magnetictunnel junction.

The substrate 100 may be a semiconductor substrate. For example, as thesubstrate 100, a silicon substrate, a gallium arsenide substrate, asilicon germanium substrate, a silicon oxide film substrate, or the likemay be used. In this embodiment, a silicon substrate is used. Inaddition, a selection element including a transistor may be formed onthe substrate 100. An insulating layer 105 may be formed on thesubstrate 100. That is, the insulating layer 105 may be formed to coverpredetermined structures such as a selection element and the like, andmay be provided with a contact hole exposing at least a part of theselection element. The insulating layer 105 may be formed using asilicon oxide (SiO₂) film having an amorphous structure.

The lower electrodes 110 are formed on the insulating layer 105. Thelower electrodes 110 may be formed using conductive materials, such asmetals and metal nitrides. In addition, the lower electrodes 110 of thepresent invention may have a dual structure in which first and secondlower electrodes 112 and 114 are formed. In this case, the first lowerelectrode 112 may be formed on the insulating layer 105, and the secondlower electrode 114 may be formed on the first lower electrode 112. Inaddition, the first lower electrode 112 may be formed in the insulatinglayer 105, and may be connected to the selection element formed on thesubstrate 100. The first and second lower electrodes 112 and 114 may beformed of a polycrystalline material. That is, the first and secondlower electrodes 112 and 114 may be formed of a conductive materialhaving a bcc structure. For example, the first lower electrode 112 maybe formed of a metal such as tungsten (W), and the second lowerelectrode 114 may be formed of a metal nitride film such as a titaniumnitride (TiN) film.

The buffer layer 120 is formed on the upper part of the lower electrodes110. That is, the buffer layer 120 is formed on the second lowerelectrode 114. The buffer layer 120 may be formed of a material havingexcellent compatibility with the second lower electrode 114 to eliminatelattice constant mismatch between the second lower electrode 114 and theseed layer 130. For example, when the second lower electrode 114 isformed of TiN, the buffer layer 120 may be formed using tantalum (Ta)having excellent lattice compatibility with TiN. In this case, Ta isamorphous, but the second lower electrode 114 is polycrystalline.Therefore, the amorphous buffer layer 120 may grow in the crystaldirection of the polycrystalline second lower electrode 114, and thencrystallinity may be improved by heat treatment. In addition, the bufferlayer 120 may be formed to have a thickness of, for example, 2 nm to 10nm.

The seed layer 130 is formed on the upper part of the buffer layer 120.The seed layer 130 may be formed of a polycrystalline material, forexample, tungsten (W). When the seed layer 130 is formed of apolycrystalline material, the crystallinity of a magnetic tunneljunction including the free layer 140, the tunneling barrier 150, andthe pinned layer 160 formed on the seed layer 130 may be improved. Thatis, when the polycrystalline seed layer 130 is formed, the amorphousmagnetic tunnel junction formed on the seed layer 130 grows in thecrystal direction of the seed layer 130. Then, when heat treatment isperformed to impart perpendicular magnetic anisotropy, the crystallinityof the magnetic tunnel junction may be improved as compared with aconventional method. In particular, when W is used as the seed layer130, crystallization occurs after heat treatment is performed at a hightemperature of 400° C. or higher, for example, 400° C. to 500° C.Therefore, Ta diffusion into a tunnel barrier 150 is suppressed, and thefree layer 140 and the pinned layer 160 may crystallize, thusmaintaining the perpendicular magnetic anisotropy of the magnetic tunneljunction. That is, in the conventional method, since an amorphous seedlayer and an amorphous magnetic tunnel junction are formed on anamorphous insulating layer, crystallinity is not improved even afterheat treatment. However, according to the present invention, thecrystallinity of a magnetic tunnel junction may be improved. In thiscase, when a magnetic field is applied, large magnetization may begenerated, and the amount of current flowing through the magnetic tunneljunction in a parallel state may be increased. Therefore, when such amagnetic tunnel junction is applied to a memory device, the operatingspeed and reliability of the memory device may be improved. In addition,the seed layer 130 may be formed to have a thickness of, for example, 1nm to 3 nm.

The free layer 140 is formed on the seed layer 130, and is formed of aferromagnetic material. In the free layer 140, the direction ofmagnetization is not fixed to one direction, but may be switched fromone direction to the opposite direction. That is, the magnetizationdirection of the free layer 140 may be the same (that is, parallel) asor opposite (that is, antiparallel) to the magnetization direction ofthe pinned layer 160. A magnetic tunnel junction may be utilized as amemory element by mapping information of ‘0’ or ‘1’ to a resistancevalue which changes depending on the magnetization arrangement of thefree layer 140 and the pinned layer 160. For example, when themagnetization direction of the free layer 140 is parallel to that of thepinned layer 160, the resistance value of a magnetic tunnel junctionbecomes small. In this case, data may be defined as ‘0’. In addition,when the magnetization direction of the free layer 140 is antiparallelto that of the pinned layer 160, the resistance value of a magnetictunnel junction becomes large. In this case, data may be defined as ‘1’.For example, the free layer 140 may be formed using an amorphousrare-earth element alloy, a multilayer thin film in which a magneticmetal and a non-magnetic metal are alternately stacked, an alloy havingan L10 type crystal structure, or a ferromagnetic material such as acobalt alloy. As the amorphous rare-earth element alloy, alloys such asTbFe, TbCo, TbFeCo, DyTbFeCo, and GdTbCo may be used. In addition, themultilayer thin film in which a magnetic metal and a non-magnetic metalare alternately stacked includes Co/Pt, Co/Pd, CoCr/Pt, Co/Ru, Co/Os,Co/Au, Ni/Cu, CoFeAl/Pd, CoFeAl/Pt, CoFeB/Pd, CoFeB/Pt, and the like. Inaddition, the alloy having an L10 type crystal structure includesFe50Pt50, Fe50Pd50, Co50Pt50, Fe30Ni20Pt50, Co30Ni20Pt50, and the like.In addition, the cobalt alloy includes CoCr, CoPt, CoCrPt, CoCrTa,CoCrPtTa, CoCrNb, CoFeB, and the like. Among these materials, the CoFeBsingle layer may be formed thicker than CoFeB and Co/Pt or Co/Pd of amultilayer structure, and thus the magnetoresistance ratio may beincreased. In addition, since CoFeB is easier to etch than metals suchas Pt and Pd, the CoFeB single layer is easier to fabricate than amultilayer structure containing Pt or Pd. Thus, according to anembodiment of the present invention, a CoFeB single layer is used toform the free layer 140, and the CoFeB is formed into an amorphous stateand then textured into a BCC (100) by heat treatment.

The tunneling barrier 150 is formed on the free layer 140 and separatesthe free layer 140 and the pinned layer 160. The tunneling barrier 150allows quantum mechanical tunneling between the free layer 140 and thepinned layer 160. The tunneling barrier 150 may be formed of magnesiumoxide (MgO), aluminum oxide (Al₂O₃), silicon oxide (SiO₂), tantalumoxide (Ta₂O₅), silicon nitride (SiNx), aluminum nitride (AlNx), or thelike. In an embodiment of the present invention, polycrystallinemagnesium oxide is used as the tunneling barrier 150. The magnesiumoxide is textured into BCC (100) by heat treatment.

The pinned layer 160 is formed on the tunnel barrier 150. In the pinnedlayer 160, magnetization may be fixed to one direction in a magneticfield within a predetermined range, and may be formed of a ferromagneticmaterial. For example, the magnetization direction may be fixed to adirection extending from the upper part to the lower part. For example,the pinned layer 160 may be formed of an amorphous rare-earth elementalloy, a multilayer thin film in which a magnetic metal and anon-magnetic metal are alternately stacked, or a ferromagnetic materialsuch as an alloy having an L10 type crystal structure. In this case, thepinned layer 160 may be formed of the same ferromagnetic material as thefree layer 140. Specifically, the pinned layer 160 may be formed of aCoFeB single layer. The CoFeB is formed into an amorphous state and thentextured into BCC (100) by heat treatment.

The capping layer 170 is formed on the pinned layer 160 and magneticallyseparates the pinned layer 160 and the SyAF layers 180 from each other.The capping layer 170 is formed so that the magnetizations of the SyAFlayers 180 and the pinned layer 160 are generated independently of eachother. In addition, the capping layer 170 may be formed in considerationof the magnetoresistance ratio of the free layer 140 and the pinnedlayer 160 for operation of a magnetic tunnel junction. The capping layer170 may be formed of a material allowing crystal growth of the SyAFlayers 180. That is, the capping layer 170 allows first and secondmagnetic layers 181 and 183 of the SyAF layers 180 to grow in a desiredcrystal direction. For example, the capping layer 170 may be formed of ametal that facilitates crystal growth in the (111) direction of aface-centered cubic (FCC) lattice or the (001) direction of a hexagonalclose-packed (HCP) structure. The material used to form the cappinglayer 170 may include tantalum (Ta), ruthenium (Ru), titanium (Ti),palladium (Pd), platinum (Pt), magnesium (Mg), cobalt (Co), aluminum(Al), and tungsten (W) and alloys thereof. Preferably, the capping layer170 may be formed of at least one of tantalum (Ta) and tungsten (W).That is, the capping layer 170 may be formed of tantalum (Ta) ortungsten (W), and may be formed as a laminated structure of Ta/W. Inaddition, the capping layer 170 may be formed to have a thickness of 0.3nm to 0.6 nm. When Ta is used, the capping layer 170 may be formed tohave a thickness of 0.4 nm to 0.6 nm. When W is used, the capping layer170 may be formed to have a thickness of 0.35 nm to 0.55 nm. In thiscase, the pinned layer 160 and the first magnetic layer 181 of the SyAFlayers 180 must be ferro-coupled to fix the magnetization direction ofthe pinned layer 160. However, when the capping layer 170 having athickness of 0.55 nm or more is formed using W, the magnetizationdirection of the pinned layer 160 is not fixed and has the samemagnetization direction as the free layer 140 due to increase in thethickness of the capping layer 170. As a result, the same magnetizationdirection and other magnetization directions required in a MRAM devicedoes not occur, and thus the MRAM device does not operate as a memory.

The SyAF layers 180 are formed on the capping layer 170. The SyAF layers180 serve to fix the magnetization of the pinned layer 160. The SyAFlayers 180 include the first magnetic layer 181, a non-magnetic layer182, and a second magnetic layer 183. That is, in the SyAF layers 180,the first magnetic layer 181 and the second magnetic layer 183 areantiferromagnetically coupled via the non-magnetic layer 182. In thiscase, the magnetization directions of the first magnetic layer 181 andthe second magnetic layer 183 are arranged antiparallel. For example,the first magnetic layer 181 may be magnetized in the upward direction(that is, the direction toward the upper electrode 190), and the secondmagnetic layer 183 may be magnetized in the downward direction (that is,the direction toward the magnetic tunnel junction). The first magneticlayer 181 and the second magnetic layer 183 may be formed by alternatelystacking a magnetic metal and a non-magnetic metal. A single metalselected from the group consisting of iron (Fe), cobalt (Co), and nickel(Ni) or an alloy thereof may be used as the magnetic metal, and a singlemetal selected from the group consisting of chromium (Cr), platinum(Pt), palladium (Pd), iridium (Ir), rhodium (Rh), ruthenium (Ru), osmium(Os), rhenium (Re), gold (Au), and copper (Cu) or an alloy thereof maybe used as the non-magnetic metal. For example, the first magnetic layer181 and the second magnetic layer 183 may be formed of [Co/Pd]n,[Co/Pt]n, or [CoFe/Pt]n (wherein n is an integer of 1 or more). Thenon-magnetic layer 182 is formed between the first magnetic layer 181and the second magnetic layer 183, and is formed of a nonmagneticmaterial that allows the first magnetic layer 181 and the secondmagnetic layer 183 to form diamagnetic coupling. For example, thenon-magnetic layer 182 may be formed of a single metal selected from thegroup consisting of ruthenium (Ru), rhodium (Rh), osmium (Os), rhenium(Re), and chromium (Cr) or an alloy thereof.

The upper electrode 190 is formed on the SyAF layers 180. The upperelectrode 190 may be formed using a conductive material such as metals,metal oxides, and metal nitrides. For example, the upper electrode 190may be formed of a single metal selected from the group consisting oftantalum (Ta), ruthenium (Ru), titanium (Ti), palladium (Pd), platinum(Pt), magnesium (Mg), and aluminum (Al) or an alloy thereof.

As described above, in the memory device according to the embodiments ofthe present invention, the lower electrodes 110 are formed of apolycrystalline material, the seed layer 130 of a polycrystallinematerial is formed thereon, and the buffer layer 120 is formed betweenthe lower electrodes 110 and the seed layer 130 to improve compatibilitybetween the lower electrodes 110 and the seed layer 130. The lowerelectrodes 110 and the seed layer 130 are formed of a polycrystallinematerial, so that an amorphous magnetic tunnel junction formed thereonis aligned with the crystal structure of the seed layer 130. After theamorphous magnetic tunnel junction is formed, heat treatment isperformed so that the amorphous magnetic tunnel junction has a bettercrystal structure than that of the conventional junction. That is, inthe perpendicular magnetization-type magnetic tunnel junction, it isimportant to texture the (100) direction of the body-centered cubic(BCC) structures of the free layer, the tunneling barrier, and thepinned layer. In the present invention, the lower electrodes are formedof polycrystalline TiN, and a polycrystalline W seed layer is formedthereon, thereby securing an interface of a flat seed layer andimproving the BCC (100) crystallinity of the magnetic tunnel junction.In addition, according to the present invention, since the seed layer130 and the capping layer 170 are each made of a metal includingtungsten, a magnetic tunnel junction is capable of maintainingperpendicular magnetic anisotropy even at a temperature of about 400° C.That is, after the upper electrode 190 is formed, metal line andpassivation processes are performed at a temperature of about 400° C. Atthis time, in a conventional case using tantalum (Ta) as the seed layer,the perpendicular magnetic anisotropy of a magnetic tunnel junction isdeteriorated at 400° C., but in the present invention, the perpendicularmagnetic anisotropy of a magnetic tunnel junction may be maintained. Thecharacteristics of the memory device according to one embodiment of thepresent invention and a conventional memory device are as follows.

FIG. 2 is a graph showing heat treatment temperature-dependentmagnetization of perpendicular magnetic anisotropy structures in which aseed layer is formed using Ta and W, respectively. First, aperpendicular magnetic anisotropy structure, in which a silicon oxidefilm, a Ta or W seed layer (5 nm), a CoFeB magnetic layer (1.05 nm), anda MgO tunneling barrier (2.0 nm) were laminated on a silicon substrate,was fabricated, and heat treatment was performed at various temperaturesranging from 275° C. and 400° C. As shown in FIG. 2, when a seed layeris formed using Ta, the magnetization value of a magnetic layer is keptconstant from 275° C. to 300° C., but the magnetization value is loweredat 300° C. or higher. That is, when a magnetic layer is formed on a Taseed layer, the perpendicular magnetic anisotropy of the magnetic layeris deteriorated at a high temperature of 300° C. or higher. However,when a seed layer is formed using W, the magnetization value ismaintained at a temperature of 400° C. That is, when a magnetic layer isformed on a W seed layer, the magnetic layer is capable of maintainingperpendicular magnetic anisotropy at a high temperature of 400° C. orhigher.

In addition, FIG. 3 is a graph showing magnetic field-dependentmagnetization when Ta is used as a seed layer, and FIG. 4 is a graphshowing magnetic field-dependent magnetization when W is used as a seedlayer. As shown in FIG. 3, when Ta is used as a seed layer,perpendicular magnetic anisotropy is maintained up to 300° C., butperpendicular magnetic anisotropy is gradually decreased from 300° C.and completely vanishes at 350° C. or higher. On the other hand, asshown in FIG. 4, when W is used as a seed layer, perpendicular magneticanisotropy is maintained even when temperature rises to 400° C.

When a CoFeB magnetic layer, that is, a free layer and a CoFeB pinnedlayer, is formed on a Ta seed layer, the CoFeB magnetic layer is formedon the amorphous lattice structure of Ta. Therefore, as shown in FIG. 5,the amorphous state of the Ta seed layer and the CoFeB magnetic layer ismaintained even when heat treatment is performed at a temperature ofabout 275° C. In addition, after heat treatment, a MgO tunnel barrierbecomes an L10 crystal (bcc like) structure, boron (B) partiallydiffuses from the free layer and the CoFeB pinned layer to the MgOtunneling barrier and a Ta capping layer, and Co atoms and Fe atomsgather at the interface of MgO/CoFeB. Thereafter, CoFeB is maintained inan amorphous state, but at the interface of MgO/CoFeB, oxygen of MgO andFe of CoFeB are combined to form Fe—O bonds. Fe—O bonding is animportant mechanism for maintaining perpendicular properties becauseFe—O bonding allows formation of interfacial perpendicular magneticanisotropy (interfacial PMA). However, as shown in FIGS. 6 and 7, afterheat treatment at 325° C. and 350° C., the Ta seed layer passes throughthe amorphous CoFeB and diffuses into the MgO tunnel barrier, therebydeteriorating the crystalline linearity of MgO. At this time, the atomicradius of Ta (135 pm) is almost equal to the atomic radius of Mg (137pm), and therefore, MgTaOx is expected to be formed, which maydeteriorate the crystallinity of the MgO tunnel barrier.

In addition, Ta is one of transition metals which are d-orbital elementsand has a bcc lattice structure similar to the L10 lattice (bcc like)structure of the MgO tunnel barrier. In particular, a material having abcc lattice structure among transition metals is preferably used as aseed layer used in a magnetic tunnel junction. Transition metals havinga typical bcc lattice structure include V, Nb, Ta, Cr, Mo, W, Fe, andthe like. However, since Fe and Cr have a too high or lowcrystallization temperature and have a high lattice mismatch with a TiNlower electrode, Fe and Cr may not be used as a seed layer formaintaining the perpendicular magnetic anisotropy of CoFeB. On the otherhand, according to the present invention, a seed layer may be easilyformed using W as in the case of using Ta. In particular, W crystallizesafter heat treatment at a high temperature of 400° C., therebysuppressing Ta diffusion into the MgO tunnel barrier and furthercrystallizing the CoFeB magnetic layer to maintain the perpendicularmagnetic anisotropy of the magnetic tunnel junction.

FIG. 8 is a graph showing heat treatment temperature-dependentmagnetization of a perpendicular magnetic anisotropy structure in whicha tungsten seed layer is formed directly on a TiN lower electrode. Thatis, a first tungsten lower electrode and a second TiN lower electrodewere formed on a silicon substrate, and a W seed layer, a CoFeB magneticlayer (1.05 nm), and a MgO tunneling barrier (2.0 nm) were laminatedthereon, and magnetization characteristics depending on heat treatmenttemperatures were measured. In this case, the W seed layer was formed tohave a thickness of 3 nm, 4 nm, 5 nm, or 8 nm. As shown in FIG. 8, whenthe W seed layer is formed directly on the TiN lower electrode, latticemismatch between TiN and W becomes large and compatibility is reduced,which affect magnetic layer growth, so that perpendicular magneticanisotropy is not generated.

To solve these problems and form the W seed layer on the W/TiN lowerelectrode, the buffer layer is preferably formed using Ta havingcompatibility with TiN, and the W seed layer is formed thereon. FIG. 9is a graph showing heat treatment temperature-dependent magnetization ofa perpendicular magnetic anisotropy structure according to an embodimentof the present invention, in which a W/TiN lower electrode, a Ta bufferlayer, a W seed layer, a CoFeB magnetic layer (1.05 nm), and a MgOtunneling barrier (2.0 nm) are laminated on a silicon substrate. In thiscase, the W seed layer was formed to have a thickness of 1 nm, 1.5 nm, 2nm, 2.5 nm, 3 nm, or 4 nm, and heat-treated at a temperature of 400° C.As shown in FIG. 9, in the case of the present invention in which a Tabuffer layer and a W seed layer are formed on a W/TiN lower electrode,perpendicular magnetic anisotropy is evidently generated. That is, asshown in FIG. 9, when magnetization direction changes from one side tothe other side, a rough square graph is formed due to coercive force. Inthis case, in the present invention, the squareness of the square isabout 0.8 to 1. As squareness increases, the rate of magnetizationchange increases and the operating speed of a memory device may beincreased. That is, according to the present invention, the read/writeoperation time of a memory device may be reduced.

In addition, FIG. 10 is a TEM image after heat treatment in the casewherein a Ta seed layer is formed on a TiN lower electrode, and FIG. 11is a TEM image after heat treatment in the case wherein a Ta bufferlayer and a W seed layer are formed on a TiN lower electrode. That is,FIG. 10 is a TEM image of a case wherein a Ta seed layer is formed on aTiN lower electrode, a CoFeB magnetic layer and a MgO tunnel barrier areformed, and heat treatment is performed at 400° C., and FIG. 11 is a TEMimage of a case wherein a Ta buffer layer and a W seed layer are formedon a TiN lower electrode, a CoFeB magnetic layer and a MgO tunnelbarrier are formed, and heat treatment is performed at 400° C. As shownin FIG. 10, when a Ta seed layer is formed, a CoFeB magnetic layermaintains an amorphous state. On the other hand, as shown in FIG. 11, aCoFeB magnetic layer formed on a Ta buffer layer and a W seed layercrystallizes upon heat treatment at a temperature of 400° C.

In addition, to evaluate the perpendicular magnetic anisotropy of thefree layer and the pinned layer of the perpendicular magnetic tunneljunction according to the present invention, a pseudo spin valvestructure was fabricated, and magnetic field-dependent magnetization wasmeasured. The obtained results are shown in FIG. 12. FIG. 12 is a graphshowing magnetic field-dependent magnetization of the pseudo spin valveaccording to the present invention in which a silicon oxide film isformed on a silicon substrate, and a W/TiN lower electrode, a Ta bufferlayer (5 nm), a W seed layer (2 nm), a CoFeB free layer (1.05 nm), a MgOtunnel barrier (1.2 nm), a CoFeB pinned layer (1.0 nm), and a W cappinglayer (1.5 to 4 nm) are laminated thereon. That is, to confirmperpendicular magnetic anisotropy according to the thickness of a Wcapping layer, the magnetic field-dependent magnetization of pseudo spinvalves having different thicknesses of a capping layer was measured. Inthis case, as a capping layer formed on the CoFeB pinned layer, the Wcapping layer capable of maintaining the perpendicular magneticanisotropy of the CoFeB pinned layer even after heat treatment at 400°C. was used instead of Ta used in a conventional device. As shown inFIG. 2, when the W capping layer has a thickness of 4 nm or more, theperpendicular magnetic anisotropy of the CoFeB magnetic layer ismaintained. In addition, in an embodiment of the present invention, theW seed layer is formed to have a thickness of 0.35 nm to 0.55 nm, and inan experimental example, the W capping layer is formed to have athickness of 4 nm or more. In the case of the pseudo spin valvestructure according to the experimental example in which a W cappinglayer is formed to have a thickness of 4 nm or less, since no structureis formed on the capping layer, W is oxidized, which affects the CoFeBmagnetic layer, and perpendicular magnetic anisotropy characteristics isdeteriorated. Thus, the result was 4 nm or more. On the other hand,according to the embodiment, since SyAF layers are formed on the Wcapping layer, the capping layer is not oxidized. In this case, when thecapping layer is formed to have a thickness of 0.35 nm to 0.55 nm,perpendicular magnetic anisotropy may be obtained.

In the conventional case using a Ta seed layer and a Ta capping layer,CoFeB grows as an amorphous solid, and when heat treatment is performedat 300° C. or more, a Ta element diffuses into the MgO layer and thecrystallinity of MgO is destroyed. On the other hand, in the presentinvention using a W seed layer and a W capping layer, the W seed layerand the W capping layer become crystalline after heat treatment at 400°C. Therefore, W may suppress Ta diffusion into the MgO layer. Inaddition, by allowing crystallization of the CoFeB magnetic layer, acrystal-state perpendicular magnetic tunnel junction maintainingperpendicular magnetic anisotropy is formed.

In addition, in the conventional magnetic tunnel junction in which a Taseed layer and a Ta capping layer are formed, the perpendicular magneticanisotropy of a CoFeB magnetic layer was maintained upon heat treatmentat a temperature of 275° C., and the tunnel magnetoresistance (TMR)ratio was about 130.86%. However, in the conventional magnetic tunneljunction in which a Ta seed layer and a Ta capping layer are formed,when heat treatment is performed at a temperature of 300° C. or more,the perpendicular magnetic anisotropy of the CoFeB magnetic layerdisappears, and the TMR ratio is reduced. On the other hand, as shown inthe present invention, in the magnetic tunnel junction in which a Tabuffer layer and a W seed layer are formed and then a Ta capping layeris formed, a tunneling barrier was formed. In addition, when heattreatment was performed at a temperature of 400° C., the TMR ratio was136.45%, which was slightly higher than in the conventional case.However, in the case wherein the capping layer is formed using W, thatis, in a magnetic tunnel junction in which a Ta buffer layer, a W seedlayer, and a W capping layer are formed, the TMR ratio was maintained at134.44% upon heat treatment at a temperature of 400° C. The thickness ofthe W capping layer for obtaining the TMR ratio is maintained at about0.35 nm to 0.55 nm. Meanwhile, the TMR ratio was measured using acurrent-in-plane tunneling (CiPT) instrument. In the CiPT measuringmethod, two probes are bonded on a thin upper electrode, and the spacingbetween the two probes is measured and distinguished by a fewmicrometers. At this time, in the instrument, the TMR ratio iscalculated by fitting the resistances between the thin upper electrodemeasured at intervals of several micrometers and the thick lowerelectrode.

In addition, STT-MRAM stores data and functions as a memory. The spindirection of a free layer may be unintentionally changed by ambient heator other energy, which may cause the stored data to change. To solvesuch a problem, thermal stability (Δ) expressed by Equation 1 must beincreased.

$\begin{matrix}{\Delta = \frac{K_{u}V}{K_{B}T}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

In this case, the denominator is a constant that cannot be changed.Therefore, to increase Δ, the numerator value must be increased.However, perpendicular magnetic anisotropy (Ku) is an eigenvalue of amaterial, and the Ku of the free layer is limited. Therefore, a methodof increasing the volume (V) is required. In general, when interfaceperpendicular magnetic anisotropy (i-PMA) is expressed at the interfacebetween a free layer and a tunneling barrier, that is, the CoFeB/MgOinterface, the CoFeB exhibits perpendicular magnetic anisotropy (PMA)rather than horizontal magnetic anisotropy (IMA). However, when thethickness of the CoFeB is about 1.4 nm or more, horizontal magneticanisotropy (IMA), an inherent characteristic, appears. Therefore, thethickness of the CoFeB is limited, and increase in volume is alsolimited. Therefore, as shown in FIG. 13, the free layer is thickened toincrease the volume, and the thick free layer is separated so thatperpendicular magnetic anisotropy is exhibited. Other embodiments of thepresent invention will now be described with reference to FIG. 13. Inthe following description, contents overlapping with those in FIG. 1will be omitted.

Referring to FIG. 13, the memory device according to another embodimentof the present invention includes the lower electrodes 110, the bufferlayer 120, the seed layer 130, a lower barrier 155, a first free layer142, a separation layer 144, a second free layer 146, the tunnelingbarrier 150, the pinned layer 160, the capping layer 170, the SyAFlayers 180, and the upper electrode 190, which are formed on thesubstrate 100. In this case, the first free layer 142, the separationlayer 144, and the second free layer 146 form the free layers 140, andthe free layers 140, the tunneling barrier 150, and the pinned layer 160form a magnetic tunnel junction. That is, the thickness of the freelayers 140 is increased to improve thermal stability. At the same time,to prevent generation of horizontal magnetic anisotropy due to increasein the thickness of the free layers 140, the separation layer 144 isformed, so that the free layers 140 have a dual structure including thefirst and second free layers 142 and 146. In addition, since the firstfree layer 142, for example, CoFeB, exhibits perpendicular magneticanisotropy at the interface with MgO, the lower barrier 155 is formedbelow the first free layer 142.

The lower barrier 155 may be formed on the seed layer 130, and may beformed of magnesium oxide (MgO), aluminum oxide (Al₂O₃), silicon oxide(SiO₂), tantalum oxide (Ta₂O₅), silicon nitride (SiNx), or aluminumnitride (AlNx). In this case, the lower barrier 155 may be formed of amaterial identical to that of the tunneling barrier 150, or may beformed of a material different from that of the tunneling barrier 150.However, the lower barrier 155 is preferably formed of a materialidentical to that of the tunneling barrier 150, and may be formed of apolycrystalline magnesium oxide. The magnesium oxide is textured intoBCC (100) by heat treatment. The lower barrier 155 allows the first freelayer 142 to have perpendicular magnetic anisotropy.

The first and second free layers 142 and 146 are formed of aferromagnetic material. In the first and second free layers 142 and 146,magnetization may not be fixed to one direction, but may be switchedfrom one direction to the opposite direction. That is, the magnetizationdirections of the first and second free layers 142 and 146 may be thesame (that is, parallel) as or opposite (that is, antiparallel) to themagnetization direction of the pinned layer 160. In addition, themagnetization directions of the first and second free layers 142 and 146may be changed in the same direction and may be changed at the sametime. For example, the first and second free layers 142 and 146 may beformed using an amorphous rare-earth element alloy, a multilayer thinfilm in which a magnetic metal and a non-magnetic metal are alternatelystacked, an alloy having an L10 type crystal structure, or aferromagnetic material such as a cobalt alloy. For example, the firstand second free layers 142 and 146 may be formed as a CoFeB singlelayer. The CoFeB is formed into an amorphous state and then texturedinto BCC (100) by heat treatment. In addition, the first and second freelayers 142 and 146 may each have a thickness of 0.8 nm to 1.4 nm, andmay have different thicknesses or the same thickness. For example, thefirst free layer 142 may be formed to be thinner than the second freelayer 146.

The separation layer 144 is provided between the first and second freelayers 142 and 146 to separate the first and second free layers 142 and146 from each other. That is, the separation layer 144 is formed betweenthe first and second free layers 142 and 146 to prevent generation ofhorizontal magnetic anisotropy in the free layers 140 due to increase inthe thickness of the free layers 140. The separation layer 144 may beformed of a polycrystalline material. That is, the separation layer 144may be formed of a conductive material having a bcc structure. Forexample, the separation layer 144 may be formed using tungsten, and maybe formed to have a thickness of 0.4 nm to 0.8 nm. That is, theseparation layer 144 may be formed of tungsten as in the first lowerelectrode 112 and the capping layer 170. When the separation layer 144is formed of tungsten, the separation layer 144 crystallizes after heattreatment at a high temperature of 400° C. or higher, for example, 400°C. to 500° C. As a result, the free layers 140 and the pinned layer 160crystallize, so that the perpendicular magnetic anisotropy of a magnetictunnel junction may be maintained.

As described above, in the memory device according to another embodimentof the present invention, the thickness of the free layers 140 isincreased to improve thermal stability. At the same time, to preventgeneration of horizontal magnetic anisotropy due to increase in thethickness of the free layers 140, the separation layer 144 is formed, sothat the free layers 140 have a dual structure including the first andsecond free layers 142 and 146. In addition, the separation layer 144 isformed of tungsten, so that the separation layer 144 crystallizes afterheat treatment at a high temperature of 400° C. or higher. As a result,the perpendicular magnetic anisotropy of a magnetic tunnel junction maybe maintained.

The technical idea of the present invention has been particularlydescribed through the aforementioned examples. However, it should benoted that the examples were provided for explanation and the presentinvention is not limited thereto. In addition, those skilled in the artwill understand that various modifications are possible within the rangeof the technical idea of the present invention.

The invention claimed is:
 1. A memory device in which lower electrodes,a buffer layer, a seed layer, a magnetic tunnel junction, a cappinglayer, synthetic antiferromagnetic (SyAF) layers, and an upper electrodeare formed on a substrate in a laminated manner, wherein the magnetictunnel junction comprises free layers consisting of first and secondfree layers and a separation layer, and a tunneling barrier and a pinnedlayer, wherein the separation layer is formed between the first andsecond free layers and is formed of a polycrystalline conductivematerial.
 2. The memory device according to claim 1, further comprisinga lower barrier formed between the seed layer and the first free layer.3. The memory device according to claim 1, wherein the lower barrier isformed of a material identical to that of the tunneling barrier.
 4. Thememory device according to claim 1, wherein the separation layer isformed of a material which crystallizes at a temperature of 400° C. orhigher.
 5. The memory device according to claim 4, wherein theseparation layer is formed of tungsten.
 6. The memory device accordingto claim 5, wherein the separation layer is formed to have a thicknessof 0.4 nm to 0.8 nm.
 7. The memory device according to claim 2, whereinthe first and second free layers are formed to have identicalthicknesses or different thicknesses.
 8. The memory device according toclaim 4, wherein at least one of the lower electrodes, the seed layer,and the capping layer is formed of a material which crystallizes at atemperature of 400° C. or higher.
 9. The memory device according toclaim 8, wherein at least one of the lower electrodes, the seed layer,and the capping layer is formed of tungsten.
 10. The memory deviceaccording to claim 1, wherein the magnetic tunnel junction has asquareness of magnetization change of more than 0.8 and 1 or less afterheat treatment at 400° C. or higher.
 11. A memory device in which lowerelectrodes, a buffer layer, a seed layer, a magnetic tunnel junction, acapping layer, synthetic antiferromagnetic (SyAF) layers, and an upperelectrode are formed on a substrate in a laminated manner, wherein themagnetic tunnel junction comprises free layers consisting of first andsecond free layers and a separation layer, and a tunneling barrier and apinned layer, wherein the separation layer is formed between the firstand second free layers; and at least one of the lower electrodes, theseed layer, the separation layer, and the capping layer is formed of apolycrystalline conductive material.
 12. The memory device according toclaim 11, wherein the polycrystalline conductive material comprises amaterial which crystallizes at a temperature of 400° C. or higher. 13.The memory device according to claim 12, wherein the polycrystallineconductive material comprises tungsten.
 14. A method of fabricating amemory device, comprising: a step of forming lower electrodes, a bufferlayer, a seed layer, a magnetic tunnel junction, a capping layer,synthetic antiferromagnetic (SyAF) layers, and an upper electrode on asubstrate in a laminated manner; and a step of performing heat treatmentat 400° C. to 500° C., wherein, when the magnetic tunnel junction isformed, a first free layer, a separation layer, and a second free layerare formed and then a tunneling barrier and a pinned layer are formed,wherein the separation layer is formed of a polycrystalline conductivematerial; and the heat treatment is performed after the upper electrodeis formed.